Various method for measuring the size, shape and accuracy of placement of semi-conductor features, while they are still in the form of wafers of silicon are known in the art. Electron-beam imaging has been established as the technology of choice for process development and off-line quality assurance applications, as discussed in the following articles:
M. T. Postek and C. C. Joy, "Submicrometer microlectronics dimensional metrology: scanning electron microscopy," J. of Research of NBS, Vol 92(3), 205-228, 1987.
T. Ahmed, S. R. Chen, H. M. Naguib, T. A. Brunner and S. M. Stuber, "Low voltage SEM metrology for pilot line application," SPIE 775, 80-88, 1987.
As integrated circuits are fabricated with feature geometries measuring a micron or less, high resolution measurements of device features on wafers must be made inprocess, as discussed in the article, "Use of scanning electron microscope for critical dimension measurements on a semiconductor production line," P. W. Grant, SPIE 565, 169-172, 1985.
It is known in the art that sample charging may dramatically distort the line width measurement of non-conducting materials, and that the line width measurement is very senstive to lack of focus. Both effects impair the ability to measure line width automatically. It is also known in the art that any line width in the micron or submicron range, such as resist line width, cannot be expressed as a single value, but must be described by at least two important points along the line, such as the location of both the bottom and top of the resist line.
U.S. Pat. No. 4,588,890 discloses an apparatus and method for composite image formation using a scanning electron beam which employs two oppositely disposed electron detectors to detect secondary electrons emitted from a planar specimen surface after it is radiated with a primary electron beam. As the primary electron beam is scanned across a topographical feature on the specimen surface, the electron detectors are alternately switched on and off to produce first and second electric signals which are then combined to produce a composite electric signal. The signal provides an enhanced symmetrical image of the junctures of the planar surface with the opposite lower edges of the topographical feature.
To attract secondary electrons, the electron detectors are charged with a predetermined amount of voltage, which causes a deflection of the primary electron beam towards an electron detector which is currently operative. By alternately switching the pair of electron detectors on and off, the primary electron beam does not maintain a constant position, thus introducing a possible source of inaccuracy in the resulting measurements.
Another possible source of inaccuracy stems from requiring two scans per measurement. Since measurement systems invariably vibrate and since the precision requirements are in the tens of nanometers range, vibrations occurring during the two scans of a measurement will introduce significant errors.
Additionally, the invention disclosed in U.S. Pat. No. 4,588,890 only measures the junctures of a feature with the planar surface of a speciment.
G. Matsuoka et al, in "Automatic electron beam metrology system for development of very large-scale integrated devices," Journal of Vacuum Science Technology, B5(1), p. 79-83, 1987, disclose a system which uses a low voltage electron beam, a precise X-Y stage for positioning a specimen, and four electron detectors placed at the 90.degree. points around a circle. The system collects data from all four detectors during a scan of the primary electron beam and combines them as follows: a sum of all four signals (X.sup.+, X.sup.-, Y.sup.+, and Y.sup.-), a left-side signal (X.sup.+ +Y.sup.+ +Y.sup.-), and a right-side signal ((X.sup.- +Y.sup.+ +Y.sup.-). The three signals are then differentiated and a max-detection process of the larger value of the left- and right-side signals, at each point, also occurs.
The system described by G. Matsuoka et al has a few drawbacks. First, topographical information contained in each signal is potentially lost in the max-detection process. Additionally, inaccuracies due to vibrations of the X-Y stage can occur during a scan of the primary beam if the scan speed is slower than the vibration speed.